1. Field of the Invention
The present invention relates to a zinciferous composite electroplated steel sheet, and more particularly to a high corrosion resistance zinciferous composite electroplated steel sheet containing codeposited particles in a zinc plating layer.
2. Description of the Prior Arts
Two prior art methods of improving corrosion resistance of zinc plated steel sheet are as follows;
(a) A method of thickening a plating layer;
(b) A method of improving the corrosion resistance of a plating layer by converting the plating layer to an alloy or composite layer.
Out of the two methods, the method (a) has disadvantages from a viewpoint of a resource-saving and an energy-saving in a manufacturing process and further from a viewpoint of the quality of products such as a weldability, a press workability and the like. Hitherto, the method (b) has mainly been studied and developed.
In a process belonging to the method (b), zinc alloy electroplating has been widely studied and developed because alloy plating layers of zinc with other various sorts of metals can be easily effected by means of the electroplating. As a result, in alloy plating such as Zn-Co-Cr, Zn-Fe, Zn-Ni, Zn-Mn and the like, alloying elements repress the activity of zinc. This characteristics indicates improvement in a high corrosion resistance of a plating layer can be attained. Therefore, the alloy plating is already been put into practice. Besides the above zinc alloy plating, composite electroplating method is being studied wherein the particles contained in plating solutions are codeposited into plated metals mainly composed of zinc.
A composite plated steel sheet in which aluminium disperses and a method of manufacturing the same are disclosed, for example, in a Japanese Examined Patent Publication No. 30649/79. Aluminium is dispersed in electrolytically deposited zinc layers of a composite electroplated steel sheet within its range of from 1.5 to 70 wt.%. The composite electroplated steel sheet is manufactured by adding aluminium powder to a zinc plating solution to form a composite plating solution suspending the aluminium powder in the composite plating solution and an electrolizing the solution while the solution is stirred.
It is shown in a Japanese Examined Patent Publication No. 38480/85 that at least one of particles of silica sol, titanium oxide sol and zirconia sol of 100 nm or less in size which has been treated to be charged positively are used, that steel is made cathodic in acid zinc plating bath of pH 4 or less to which 1 to 200 g/l of said particles are added, and that the acid zinc plating bath is electrolized to codeposit zinc and said particles on the surface of the steel.
In a Japanese Examined Patent Publication No. 6758/87 a high corrosion resistance zinc-alumina composite electroplated steel sheet is disclosed. Its electroplated zinc layers contain 0.01 to 3.0 wt.% of alumina sol, calculated in terms of Al.sub.2 O.sub.3, which is soluble in more than 0.1N of a hydrochloric acid in concentration.
It is shown in a Japanese Examined Patent Publication No. 6760/87 that at least one of Ni.sup.2+, Fe.sup.2+ and Co.sup.2+ are adsorbed by at least one of oxide particles such as SiO.sub.2, TiO.sub.2, ZrO.sub.2, Nb.sub.2 O.sub.5 and Ta.sub.2 O.sub.5 to let the oxide particles be charged positively for the purpose of electroplating a steel sheet by dispersing stably these particles in a zinc plating bath to form a composite plating bath.
In the mentioned prior art methods, however, it is not easy to codeposit the particles dispersing in a plating bath together with zinc. Therefore, it is insufficient to optimize a composition of a composite plating layer.